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The knock metric for controlled auto-ignition (CAI) engines is assessed by considering the physical processes that establish the pressure wave that contributes to the acoustic radiation of the engine, and by analyzing pressure data from a CAI engine. Data sets from the engine operating with port fuel injection, early direct injection and late direct injection are used to monitor the effect of mixture composition stratification. Thermodynamic analysis shows that the local pressure rise produced by heat release has to be discounted by the work spent in acoustic expansion against the ambient pressure to properly predict the pressure wave amplitude. Based on this analysis, a modified correlation between the pressure wave amplitude and the maximum pressure rise rate (MPRR) is developed by introducing an MPRR offset to account for the expansion work.